This invention relates to a diester more particularly to a diester of a bisphenol and a dicarboxylic acid which has utility as a processing aid additive to linear aromatic polyesters. It also includes compositions of enhanced processability and hydrolytic stability comprising a linear aromatic polyester and the diester.
Linear aromatic polyesters derived from dicarboxylic acids (especially aromatic dicarboxylic acids) and bisphenols are well known for their suitability for molding, extrusion, casting, and film-forming applications. For example, U.S. Pat. No. 3,216,970 to Conix, discloses linear aromatic polyesters prepared from isophthalic acid, terephthalic acid, and a bisphenolic compound. Such high molecular weight compositions are known to be useful in the preparation of various films and fibers. Further, these compositions, when molded into useful articles using conventional techniques, provide properties superior to articles molded from other linear polyester compositions. For instance, aromatic polyesters are known to have a variety of useful properties, such as good tensile, impact, and bending strengths, high thermal deformation and thermal decomposition temperatures, resistance to UV irradiation, and good electrical properties.
While the aforementioned polyesters comprising bisphenol and dicarboxylic acid residues can be processed with present day conventional processing equipment, the operating conditions in the latter required for processing these polyesters is often near the operating limits of the processing equipment entailing costly expenditures of energy in processing the polyesters.
A method known to the art to overcome such difficulties in regard to processing polymers is to incorporate in the polymers processing aid additive. In the case of the present polyesters, many of the processing aid additives employed for other polymers, including other types of polyester, do not provide sufficient enhancement of processability and/or are incompatible with the polyester. In addition, many of the known processing aid additives employed in the art are polymers themselves and hence are relatively difficult and costly to prepare.
In order to form a successful molding resin on a commercial scale, a polymer should be capable of being molded conveniently without significant degradation in physical properties. In this respect, although the aforementioned aromatic polyesters generally display excellent physical and chemical properties, a persistant problem has been their sensitivity to hydrolytic degradation at elevated temperatures. This sensitivity to the combined effects of heat and moisture is also exhibited in commercially available polycarbonate resins as evidenced by the desirability of reducing the water contact of the resin to less than about 0.05% prior to molding. Unfortunately, however, the aromatic polyester resins often display a more pronounced tendency to repidly degrade and embrittle than do polycarbonate resins. This is demonstrated by the loss of tensile strength which can occur when an aromatic polyester resin is molded and subsequently immersed in boiling water. This tendency may be explained, in part, by the hydrolysis of the ester linkages under these conditions. In any event, it is to be appreciated that sensitivity to moisture represents a significant problem in aromatic polyester resins that would limit their commercial utility in applications such as in autoclaves or at elevated temperatures in humid atmospheres.
It is the principal object of this invention to provide a non-polymeric processing aid and hydrolytic stability enhancement agent for linear aromatic polyester substrates which comprise bisphenol and dicarboxylic acid residues.
It is another object of the invention to provide easily processable compositions of improved hydrolytic stability comprising said linear polyester and the processing aid additive.